In an HCCI engine, the phasing of auto-ignited combustion strongly depends on the cylinder charge temperature, composition, and the cylinder pressure at intake valve closing. Hence, the input to the engine, such as amount of fuel, fuel injection timing, spark timing, EGR valve opening, and intake and exhaust valve profiles, must be carefully coordinated to ensure that those key cylinder variables are within a range where the auto-ignited combustion can be robustly achieved. Among those inputs, the most effective input that has a significant impact on cylinder variables is the intake and exhaust valve profiles, and there are two prevailing valve strategies concerned with those profiles; exhaust recompression and exhaust re-breathing.
In the exhaust recompression strategy, the cylinder charge temperature is controlled by trapping hot exhaust gas from the previous engine cycle by closing the exhaust valve early during the exhaust stroke, while opening the intake valve with late timing symmetrical to the exhaust valve closing timing. In this valve strategy, the cylinder charge composition and temperature depend on how early the exhaust valve closes during the exhaust stroke. For example, if the exhaust valve closes earlier during the exhaust stroke, more hot exhaust gas from the previous engine cycle would be trapped in the cylinder, leaving less cylinder volume for the fresh air mass, thereby, increasing the cylinder temperature, while decreasing the cylinder oxygen level. In the exhaust recompression strategy, the exhaust valve closing timing (thereby, the intake valve opening timing) is typically quantified by valve overlap, which has a negative number. The Negative Valve Overlap (NVO) is defined as the duration in crank angle between exhaust valve closing and intake valve opening.
In the exhaust re-breathing strategy, the cylinder charge temperature is controlled by re-inducting the hot exhaust gas into the cylinder through re-opening of the exhaust valve during the intake stroke. In this valve strategy, the cylinder charge composition and temperature depend on the lift of the exhaust valve re-opening during the intake stroke. Similarly with the exhaust recompression strategy, if the exhaust valve re-opens higher during the intake stroke, more hot exhaust gas from the previous engine cycle would be re-inducted in the cylinder, leaving less cylinder volume for the fresh air mass which, as a result, would increase the cylinder temperature, while decreasing the cylinder oxygen level.